1. Field of the Invention
The present invention relates generally to prevention of crosstalk occurring among channels of a WDM (Wavelength Division Multiplexing) system, and more particularly to an optical transmitter, a terminal-station apparatus having the optical transmitter and an optical communication system employing the terminal-station apparatus.
2. Description of the Related Art
As a terminal-station apparatus for implementing WDM of a plurality of wavelength channels, there is known a terminal-station apparatus comprising a plurality of optical transmitters for outputting optical signals with wavelengths different from each other and an optical multiplexer for carrying out WDM on the optical signals output by the optical transmitters and outputting an WDM signal light as a result of the WDM. The WDM signal light is transmitted to another terminal-station apparatus by an optical-fiber transmission line. By applying the WDM to a plurality of wavelength channels in this way, an optical-fiber communication system with a large transmission capacity can be implemented.
In an optical-fiber communication system to which the WDM is applied, the smaller the gap between two adjacent wavelength channels, the greater the number of wavelength channels that can be included in a given wavelength band and, inadvertently, the larger the amount of crosstalk occurring among the wavelength channels. Accordingly, there is a demand for reduction of the amount of crosswalk occurring among the wavelength channels which is observed as a result of decreasing the gap between two adjacent wavelength channels in an attempt to increase the transmission capacity of the optical-fiber communication system.
Each of the optical transmitters has a light-source unit for generating an optical signal for one channel. In general, the light-source unit has a laser diode to undergo direct or indirect modulation. The oscillation frequency of the laser diode is determined by the temperature of the laser diode and a driving current supplied thereto. Due to the characteristic of the laser diode described above, the wavelength of an optical signal output by the optical transmitter is prone to instability. For example, at a cold start of the optical-fiber communication system or due to a trouble related to the temperature control of the laser diode or other causes, the wavelength of an optical signal generated by the optical transmitter for a certain channel may deviate from a target wavelength assigned to the channel, giving rise to crosswalk occurring among channels. If crosswalk is generated among channels, the terminal-station apparatus receiving the WDM signal light is not capable of demodulating transmitted data with a high degree of accuracy.